LONG-TERM HYDROLOGIC IMPACT ASSESSMENT Overview: Basic Curve Number Analysis The L-THIA LID Model...

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LONG-TERM HYDROLOGIC IMPACT ASSESSMENT

Overview:Basic Curve Number Analysis

The L-THIA LID Model

What is Low Impact Development

Specific LID Practices

Web-based Low Impact Development Decision Support and Planning Tool

Tutorial Outcomes

• User will employ L-THIA LID model to estimate environmental benefits for various LID practices

• User will be able to estimate costs of potential “Lot Level” LID practices

Purdue University is an Equal Opportunity/Equal Access institution.

40% 10%

50%Natural Cover

15%

55%30%

75-100% Impervious

35%

30%

35% 35-50% Impervious

Land Use Decisions Affect Runoff, Recharge, and Water Quality


Impact on Hydrology

Pre-development Post-development


Impact on Hydrology


Time

Dir

ect

Ru

nof

f (V

olu

me) Urbanized Area

Detention basin

Natural

Eventually, “increase” in1. Peak flow2. Direct runoff

after urbanization

Basic L-THIA Model

• Long-Term Hydrologic Impact Assessment– Average annual runoff– NPS pollution

• An overview / screening model

• User friendly tool

• Does not require detailed data input

• Identifies need for more detailed modeling

• Provides "What-If" alternatives evaluation


Assumptions

• Water flows across the surface to form watershed – no storm drains

• No routing of runoff• Average antecedent moisture

– soil is not saturated or frozen• Rainfall is evenly spread in local area


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Curve Number to Compute Runoff• The Curve Number (CN) relationship was originally described

in the Soil Conservation Service publication “TR-55” (NRCS, 1986) and several modifications have since been proposed.

• The relationship between rainfall, runoff and CN value is non-linear, meaning that small changes in land use or rainfall can produce large changes in runoff.

• The use of the CN equation in L-THIA LID is a simple alternative to more complicated hydrological models that require extensive data inputs which are often not readily available for most areas, or too complex.


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Nonpoint Source Calculations

• The coefficients or Event Mean Concentration (EMC) data used for the nonpoint source (NPS) water quality calculations was compiled by the Texas Natural Resource Conservation Commission (Baird and Jennings, 1996) from numerous literature and existing water quality data.

• NPS pollutant masses are computed by multiplying runoff depth for a land use by the area of that land use and the appropriate EMC value and converting units.

• A complete list of the EMC values used in the L-THIA LID model is available in Appendix B1.


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• The land uses originally proposed by Baird and Jennings have been modified for the Midwest, and consist of the following:

L-THIA LID Land Uses:• Commercial• Industrial• High Density Residential (1/4, 1/8 acre lots)• Low Density Residential (1/2, 1, and 2 acre lots)• Water / wetlands• Grass pasture• Agricultural• Forest


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• EMC values for calculation of average concentration of NPS contaminants from each landuse


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Nonpoint Source Pollution

L-THIA LID produces Average Annual Pollutant Results for sediment, nutrients, a series of metals, and bacterial indicators.(e.g. lbs of Nitrogen per year)

L-THIA LID NPS Outputs:

Nitrogen ChromiumPhosphorous NickelSuspended solids BOD (Biological Oxygen

Demand)Lead COD (Chemical Oxygen

Demand)Copper Oil and GreaseZinc Fecal ColiformCadmium Fecal Strep


Basic Model Concepts

• Model is easy to run

• Always gives an output…

• Important to understand assumptions and limitations


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How good is L-THIA

L-THIA estimates match measured direct runoff separated from USGS daily stream flow VERY WELL


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L-THIA LID

• Based on the rainfall – land cover – runoff analysis method already used in many communities

• Input: Land Use Pattern(s) + Soils Pattern• Process: Daily Runoff and Pollutant Loading

Calculations (30 years)Microscale CN Adjustment

• Output: Average Annual Runoff and NPS loads


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Model Interface


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Model Interface


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Model Interface


Low-Impact Development (LID)

An approach to land development to mimic the pre-development site hydrology to:

1) Reduce volume of runoff2) Decentralize runoff, diffusing flows into

smaller retention/detention areas3) Improve water quality4) Encourage groundwater infiltration


LID Benefits

• Ecologically Sensible• Provides Added Values / Ecosystem Services• Economically Sustainable• Lower Costs (Construction, Maintenance &

Operation) vs. Conventional• Multifunctional Practices• Ideal for Urban Retrofit


LID Major Components

• Conservation (Watershed and Site Level) • Minimization (Watershed and Site Level) • Integrated Management Practices (Site Level)


Conservation Forest/Woods

Infiltrable Soils

Storage, Detention & Filtration Rain gardens Drainage swales Green roofs Porous Pavement

Minimization Reduce imperviousness Soil Compaction

L-THIA LID Basic Screening

Application: Target preliminary goals at the watershed and site level

• Reduce imperviousness• Conserve infiltratable soils• Conserve functional / sensitive landscape• Minimize land disturbances• Anticipate need for other LID practices to reduce

NPS and stormwater volume


L-THIA LID Available Practices

• porous pavement (narrow or pervious)• permeable or disconnected patios/sidewalks• rain barrel/cistern• green (vegetative) roof• bioretention/rain garden• grass swale• open wooded space –soil conditions


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LID Estimated Costs

Cost estimates (2008) for BMP construction.


Next: Review of LID Practices

Information sources.

Details about estimating costs of specific practices. Design information in Appendix B3.



Porous or permeable pavement, sidewalks, driveways are surfaces that infiltrate, constructed from a range of materials.

Porous Pavement –Lot Level


Specify width of streets and sidewalks,

Disconnection from stormwater system;

The % impervious of the parking lot pavement

Pavement Cost Estimate


This land use is 1/8 acre lot, high density residential.

The default assumption is 871 ft2 of pavement per driveway per lot.

To cost the porous pavement option, user assumes 8 lots this size per acre.

Cost table has range of $2 to $12 per ft2 of pavement.

Estimate could be $10.00 times 8 lots times 871 ft2 =

$ 69680 / acre

http://en.wikipedia.org/wiki/Faroe_Islands.JPG


City Hall’s Green Roof, Chicagohttp://en.wikipedia.org/wiki/Chicago_City_Hall_Green_Roof.jpg


Green Roof


Wide variation in cost and complexity

Low estimate $ 8.50 per square footHigh estimate $ 48.50 per square foot.

Next slide is an example with 28 commercial acres with 25% roof, or 7 acres of roof. That creates 43560 * 7 = 304920 ft2 feet of roof.

And this proposal is that 50% of the roof will have LID practice:Mid-range cost of $ 29.00 ft2 times 0.50 % of 304920 ft2 is$ 4.4 million.

Percent of roof treated with BMP

Total area of this landuse

Percent of area which is roof

Green Roof


Rain Barrels

• Installation of rain barrels and cisterns in residential subdivisions allows harvest of rainfall water for potential reuse.

• In many countries with water scarcity problems, especially in developing countries, the use of vertical storage systems, tanks, and underground storage structures is a common practice and serves as good water supply reservoirs.

• The value of runoff CN used to represent rain barrels is 94 and cisterns is 85 for the 4 HSGs (Sample et al., 2001).


L-THIA LID: Lot Level Screening Tool• For a median price barrel assume $200

each;

• The model assumes one per homeowner.

• High density residential 1/8 acre lot land use would require 8 barrels per acre if practice is 100%; 4 barrels /ac at 50%.

• $ 1600 per acre of 1/8 acre lot residential landuse at 100% LID.


Rain Barrels may be installed by volunteers or professionals, so costs vary.

L-THIA LID: Lot Level Screening Tool

Application: Target preliminary goals by adjusting lot level features

• Site Design & Development preparation– Narrowing impervious areas (sidewalks, driveways, roads)– Natural resource preservation– Heavy equipment use compaction– Permeable paving materials– Vegetative roof systems

• Bioretention cells• Vegetated swales /Filter strips• Rain barrels• Disconnect impervious areas


Design of bioretention systems is very site-specific.

The L-THIA LID practice is applied as a benefit (to the curve number) for the entire area.

The LID size is generalized and assumed to be adequate. A typical size (for cost estimates) for a residential lot could be 100 ft2.


Using L-THIA LID Lot Level

• Reduce street width from 26ft. to 18ft.• Rain barrels for Residential• Green Roofs for Commercial• Bioretention/Raingardens

Reduces Post-developed runoff by 46%


SummaryL-THIA LID is a screening tool to evaluate the

benefits of LID practices

L-THIA LID provides an easy to use interface

Will enable decision makers to formulate watershed management plans to meet goals

Along with other tools, allows stakeholders to understand impacts of water quantity and quality resulting from land use change

https://engineering.purdue.edu/~lthia/


https://engineering.purdue.edu/~lthia/

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DIGITAL WATERSHEDHIGH IMPACT TARGETING


Walkthrough

At the completion of this tutorial, the user should be able to design a similar scenario, enter the needed input data in L-THIA LID, run the model, and create output tables and graphs to address development questions.


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Walkthrough

Task: Use L-THIA LID to explore a 1000+ unit housing proposal for the Milan or Saline area. We will start with the assumption of 1/8 acre lot sizes on 155 acres of land. The model will produce predictions for runoff volume and NPS sediment changes in various configurations of housing unit density including LID vs. non-LID results. While local political focus is on several NPS chemistries, this tutorial’s main focus is on sediment and runoff volume.


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Walkthroughhttps://engineering.purdue.edu/~lthia/LID

The 5 part process is this: (1) Select a state and county, which determines the rainfall data for

the 30 year period.(2) Enter land use and soil data for existing conditions. (3) Enter changed land use, reflecting a proposed development. (4) Select the proportion of the area that will receive LID practices,

and may chose to select some parameters for LID practices.(5)The model runs and produces a table of outputs for examination.


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Hydrologic Soil Groups

Hydrologic soil GroupsPurdue University is an Equal Opportunity/Equal Access institution.

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Hydrologic Soil Groups

Estimate HSG based on surface texture


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Select State and county

Select Michigan then Washtenaw County.Purdue University is an Equal Opportunity/Equal Access institution.

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Select Units

Select acres as units- Note the tool tips and pop-ups over?Purdue University is an Equal Opportunity/Equal Access institution.

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Pre-Developed land use and soil

Agricultural – B Soil – 35 AcresAgricultural – C Soil – 120 Acres


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Post-Developed land use

In this scenario of a single large development, build High Density Residential 1/8 acre lot – on all the land that is being developed.


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Apply LID Percentages

Apply LID practices to 100 % of the developmentPurdue University is an Equal Opportunity/Equal Access institution.

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Select LID Screening Level

In this scenario we will do a Lot Level Screening of a single large development, with High Density Residential 1/8 acre lot – on all the land that is being developed.


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BMPs at Lot Level

Each land use has a set of controls for LID practices. In this tutorial we have one post-developed land use, “High Density Residential” and two clusters of tools – because there are two soil types.

Soil Group B

Soil Group C

Land use

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Rain BarrelsIn this first case for the Lot Level Screening, lets examine the impact of rain barrels on all the lots (full build-out of 8 per acre on all 155 acres).Check the box for Rain Barrels under each Landuse.-Remember to tab over and do it for Commercial Land use.

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After you repeat the process for both tabs (for a total of three checkboxes) click “Next”.The only changes we have made is to employ rain barrels on all lots and on the commercial development.


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Average RunoffThe model predicts that while runoff is up with the full buildout of 1240 lots, the addition of rain barrels could reduce average runoff by 10 acre-feet per year.


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Suspended SolidsThe model predicts Suspended Solids are less with the removal of ag land, with the full buildout of 1240 lots the addition of rain barrels could reduce Average Suspended Solids by 10 percent per year.


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SummaryLID Scenario Avg. Annual Runoff Volume (acre-ft)Pre-Development (existing hydrology) 34.2Post-Development without LID 90.8

LID OptionsPost-Development with Green Roof 65.0Post-Development with Rain Barrels 80.4Post-Development with Bioretention 65.1Post-Development with Porous Parking (Med.) 52.8Post-Development with Roads with Dis. Swales 76.4Post-Development with Nature Conservation Area 82.7

This table represents 6 individual runs of the model.


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DIGITAL WATERSHEDHIGH IMPACT TARGETING


Tutorial


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LONG-TERM HYDROLOGIC IMPACT ASSESSMENT Overview: Basic Curve Number Analysis The L-THIA LID Model...

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